This invention relates to the etching of aluminum foil for electrolytic capacitors using alternating current and a chloride electrolyte containing not only phosphoric acid but also alkali or alkaline earth metal chloride.
AC etching has been used to produce aluminum articles such as lithographic plates and capacitor foil. It also has been used to electropolish aluminum articles prior to metal plating.
The prior art has discussed the difficulties in obtaining an even or uniform etch structure and has overcome these difficulties in a variety of ways, e.g., interrupting the etch process to apply protective coatings, carrying out the etch process in stages of different degrees of aggressiveness, and using additives in the electrolyte bath to control pit size or to increase mechanical strength of the foil. Another problem has been to prevent the precipitation of aluminum hydroxide, formed during etching, on or into the etched surfaces.
The resolution of these problems has led to processes in which the etch conditions are carefully controlled to provide the desired increase in surface area and, particularly for capacitor foil, little change in mechanical strength. Such a process is described by M. R. Arora and J. J. Randall, Jr., in copending application Ser. No. 187,241 filed Sept. 15, 1980 now U.S. Pat. No. 4,279,714 that uses an electrolyte containing hydrochloric acid, phosphate ion and aluminum chloride.